In a synchronous machine, the magnetic field of the stator produced by the stator windings interacts with the magnetic field of the rotor, causing the rotor to rotate. The rotor magnetic field may be produced either by dc-excited rotor windings or by permanent magnets. When permanent magnets are used they may be located at the surface of the rotor or may be located in the interior of the rotor, surrounded by pole pieces. In any case, the stator typically comprises multiphase windings and is identical to the stator of an induction motor.
Examples of interior permanent magnet rotors are described in U.S. Pat. No. 4,506,181 issued to Jones et al. on Mar. 19, 1985, U.S. Pat. No. 4,472,651, issued to Jones on Sept. 18, 1984, and U.S. Pat. No. 4,469,970, issued to Neumann on Sept. 4, 1984, all three patents being assigned to the assignee of the present application. Thus, in addition to a rotor core, permanent magnets and pole pieces, an interior permanent magnet rotor may include a squirrel-cage for line starting the motor as an induction motor.
Due to the construction of the interior permanent magnet rotor there are two components to the torque produced by the motor when operating at substantially synchronous speed. First, there is field torque resulting from magnet flux and the stator current, which is present in any permanent magnet synchronous motor. Second, there is reluctance torque due to the saliency of the rotor introduced by the pole pieces. Thus, reluctance torque is present only in the interior permanent magnet motor and total torque is the sum of field torque and reluctance torque.
In view of the mechanisms of torque production, it has been known to control total torque of the interior permanent magnet motor by controlling the amplitude and the orientation of the stator current vector relative to the instantaneous orientation of the rotor magnets. However, prior art control algorithms have failed to provide the full torque producing capability of the motor at high speeds because of the saturation of the current regulated, pulse width modulated (PWM) inverter. Saturation is the phenomenon where, because of the increased back-emf generated by the rotor magnets under such conditions, the source voltage of the inverter becomes insufficient to allow accurate regulation of the instantaneous phase currents required for constant torque operation. As a result, the phase currents drop in amplitude and phase below the commanded levels.